The present invention relates to a driving method for reading out signal charges in a charge storage type solid image pickup device which is applied to electronic still cameras, and more particularly to a driving method for reading out signal charges by utilization of an electronic shutter operation.
One example of a charge storage type solid image pickup device applied to an electronic still camera is a charge storage type image pickup device of a frame interline transfer system as shown in FIG. 1.
The solid image pickup device is designed as follows. As shown in FIG. 1, a plurality of light receiving elements (or picture elements) PA.sub.11, PA.sub.12 through PA.sub.1n; PB.sub.21, PB.sub.22 through PB.sub.2n; PA.sub.31, PA.sub.31 through PA.sub.3n; PB.sub.41, PB.sub.42 through PB.sub.4n; and so forth, are formed in columns on a semiconductor substrate and are arranged in matrix form. The rows of light receiving elements PA.sub.11 through PA.sub.1n; PA.sub.31 through PA.sub.3n; and so forth, in the odd-numbered lines provide odd-numbered fields, and the rows of light receiving elements PB.sub.21 through PB.sub.2n, PB.sub.41 through PB.sub.4n; and so on, provide even-numbered fields. In FIG. 1, reference characters 1.sub.1, 1.sub.2, through 1.sub.n designate individual columns of charge transfer paths joining a respective column of the above-described light receiving elements. The upper surfaces of these charge transfer paths 1.sub.1 through 1.sub.n are covered with shielding films so as to be shielded from light.
Transfer gates G.sub.11 through G.sub.1n; G.sub.21 through G.sub.2n; G.sub.31 through G.sub.3n; G.sub.41 through G.sub.4n; and so forth are arranged in rows and connect a respective light receiving element in a corresponding row and a charge transfer path adjacent thereto. These transfer gates transfer signal charges produced in the light receiving elements to the charge transfer paths. First transmit gates comprising charge transfer elements HA.sub.11 through HA.sub.1n; HB.sub.21 through HB.sub.2n; HA.sub.31 through HA.sub.3n; HB.sub.41 through HB.sub.4n; and so forth, correspond to respective ones of the above-described transfer gates and are formed in the charge transfer paths 1.sub.1, 1.sub.2 through 1.sub.n. Second transmit gates comprising charge transfer elements LA.sub.11 through LA.sub.1n; LB.sub.21 through LB.sub.2n; LA.sub.31 through LA.sub.3n; LB.sub.41 through LB.sub.4n; and so forth, are similarly formed in the above-described charge transfer paths.
The region occupied by the light receiving elements and the charge transfer paths is a light receiving section. Provided below the light receiving section is a storage section made up only of charge transfer paths. In the charge transfer paths of the storage section, transmit gates also are provided to form the same number of charge transfer elements as in the light receiving section. The upper surface of the storage section is covered so as to be shielded from light. Drive signals V.sub.1P, V.sub.2P, V.sub.3P, and V.sub.4P of a so-called "four-phase drive system " are applied to the transmit gates in the light receiving section, and drive signals V.sub.1S, V.sub.2S, V.sub.3S and V.sub.4S of a four phase drive system are applied to the transmit gates in the storage section.
A horizontal CCD is connected to the charge transfer elements at the end of the storage section, so that signal charges transferred from the storage section are applied in series to an output amplifier in response to drive signals .phi..sub.1 and .phi..sub.2 of a so-called "two-phase drive system".
The shutter operation and the reading operation of the charge storage type solid image pickup device of the prior art will be described with reference to FIG. 6.
First, a resetting operation for eliminating unwanted signal charges is carried out. At a time instant t.sub.1, the drive signals V.sub.1P, V.sub.3P and V.sub.4P are set to the "M" level (which is between the "H" and "L" levels) and the drive signal V.sub.2P is set to the "L" level. At the following time instant t.sub.2, the drive signal V.sub.1P is raised to the "H" level for a predetermined period of time, so that the transfer gates G.sub.11, G.sub.12 through G.sub.1n; G.sub.31, G.sub.32 through G.sub.3n; G.sub.51, G.sub.52 through G.sub.5n, and so forth, of the odd-numbered fields are turned on and the unwanted signal charges of the light receiving elements PA.sub.11, PA.sub.12 through PA.sub.1n; PA.sub.31, PA.sub.32 through PA.sub.3n; PA.sub.51, PA.sub.52 through PA.sub.5n; and so forth, of the odd-numbered fields are transferred to the charge transfer elements. Next, at the time instant t.sub.3, the drive signal V.sub.3P is raised to the "H" level for a predetermined period of time. This causes the transfer gates G.sub.21, G.sub.22 through G.sub.2n; G.sub.41, G.sub.42 through G.sub.4n; G.sub.61, G.sub.62 through G.sub.6n; and so forth, to be turned on and the unwanted signal charges of the light receiving elements PB.sub.21, PB.sub.22 through PB.sub.2n; PB.sub.41, PB.sub.42 through PB.sub.4n; PB.sub.61, PB.sub.62 through PB.sub.6n; and so forth, are transferred to the charge transfer elements. During a predetermined period of time T.sub.D between the time instants t.sub.4 and t.sub.5, charge transfer of a four-phase drive system is carried out with the drive signals V.sub.1P, V.sub.2P, V.sub.3P and V.sub.4P, so that the unwanted signal charges are shifted from the light receiving section to the storage section. Thus, the resetting operation has been accomplished.
During the resetting operation, that is, the charge transfer period T.sub.D for sweeping out the above-described unwanted signals charges, all of the transfer gates are in the "off" state. In addition, all of the light receiving elements perform their photo-electric conversion operations. At the time instant t.sub.6, the drive signals V.sub.1P and V.sub.2P are set to the "M" level, and the drive signals V.sub.3P and V.sub.4P to the "L" level. At the time instant t.sub.7, the drive signal V.sub.1P is raised to the "H" level so that the signal charges produced by the light receiving elements PA.sub.11, PA.sub.12 through PA.sub.1n; PA.sub.31, PA.sub.32 through PA.sub.3n; PA.sub.51, PA.sub.52 through PA.sub.5n; and so forth, of the odd-numbered fields are transferred to the charge transfer elements. Hence, the period of time T.sub.S between the time instant t.sub.2 and t.sub.7 is a shutter period for the light receiving elements arranged in the odd-numbered fields.
Next, at the time instant t.sub.8, the drive signal V.sub.3P is raised to the "H" level, so that the signal charges produced by the light receiving elements PB.sub.21, PB.sub.22 through PB.sub.2n; PB.sub.41, PB.sub.42 through PB.sub.4n; PB.sub.61, PB.sub.62 through PB.sub.6n; and so forth, of the even-numbered fields are transferred to the charge transfer elements. Hence, the period T.sub.S between the time instants t.sub.3 and t.sub.8 is a shutter period for the light receiving elements arranged in the even-numbered fields. The period between the time instants t.sub.2 and t.sub.7 and the period between the time instants t.sub.3 and t.sub.8 are made equal to each other so that the shutter period for the light receiving elements in the odd-numbered fields and the shutter period for the light receiving elements in the even-numbered fields are equal to each other.
Next, during the predetermined period T.sub.H between the time instants t.sub.9 and t.sub.10, all the signal charges in the charge transfer elements are transferred to the storage section at high speed, and during the period T.sub.R the time instants t.sub.11 and t.sub.12 they are transferred in parallel to the horizontal CCD line by line. Next, they are transferred in series by the horizontal CCD to the output amplifier so that they are output in series by the output amplifier.
The period T.sub.O between the time instants t.sub.1 and t.sub.13 is a period for which one still picture is taken with the electronic still camera.
As is apparent from the above description, the shutter period is determined by setting the time interval T.sub.S between the transfer gate opening and closing operations.
The above-described conventional solid image pickup device suffers from the following problems. When the signal charges produced by the picture elements are transferred to the respective charge transfer elements, the signal charges of two picture elements in the odd-numbered and even-numbered fields are mixed, and only the field picture signal charges are obtained as a result. Therefore, the vertical resolution is decreased by one-half. Furthermore, since the number of the charge transfer paths in the storage section must be equal at least to the number of the charge transfer paths in the light receiving section, the area occupied by the storage section is relatively large, with the result that the semiconductor chip is unavoidably large in size.
Moreover, the conventional device suffers from field flickering. Field flickering occurs when the ratio of mixing smear components with the signal charges corresponding to one field (odd-numbered or even-numbered) is larger than that for the other field. In addition, field flickering becomes significant as the shutter speed increases because the ratio of the smear component to the signal charge increases with shutter speed.